A power converter such as an inverter controls an electric current of a multi-phase dynamoelectric machine using a pulse width modulation (PWM) scheme. When the dynamoelectric machine is a three-phase motor, an electric current of the three-phase coil is controlled by comparing voltage command values applied to each of the three-phase coils and a carries signal serving a reference of the PWM and switching an ON/OFF state of a switching element of the three-phase inverter. An output torque and a rotation speed of the three-phase motor are controlled to desired values on the basis of the currents of the three-phase coils.
A three-phase inverter widely employed in the art includes three legs, in which upper and lower arms are formed using a pair of switching elements vertically connected in series, and a pair of the arms constitute a single leg. A single leg corresponds to one of the three phases, and the three legs corresponding to the three phases have a phase difference set to an electrical angle of 120° and are controlled to output an AC voltage and an AC current of a single cycle (having an electrical angle of 360°).
As a failure occurs in the three-phase inverter, a short circuit or an open circuit is generated in the arms. The failures mean whether the switching element state for the failed arm is short-circuited or open-circuited after the failure. In the case of the open circuit, it is difficult to turn on the failed arm and apply a desired voltage. In comparison, in the state of the short circuit, the failed arm is turned on at all times. Therefore, it is difficult to turn off the failed arm, and a regenerative current flows depending on a phase of the rotor of the motor. As a result, a brake torque is generated in the motor.
Related Art 1 of PTL 1 discusses a problem regarding a brake torque of a failed channel in an inverter and a motor having a plurality of channels. As a solution, PTL 1 introduces a method of driving non-failed channels to reduce a brake torque by setting all 5169049-1 phases of the failed channels to OFF.